In recent years, development has been progressing rapidly for flat panel display devices such as liquid crystal display devices and organic EL display devices. In order to increase display quality, such flat panel display devices often have an active matrix substrate upon which a thin film transistor (also referred to as a TFT below) is provided for each of a plurality of pixels for driving the pixels.
Display devices have the active matrix substrate and an opposite substrate disposed opposite to the active matrix substrate and bonded thereto through a frame-shaped sealing member. In the display device, a display region is formed on the inner side of the sealing member, whereas a non-display region is formed in the outer periphery of the display region.
Low temperature polysilicon (also referred to as LTPS below) that has been melt-crystallized by an excimer laser is suitably used in the TFTs formed for the respective pixels in the display region. TFTs having LTPS as the active layer have the advantage of a low threshold voltage and high-speed driving, and are widely used in high definition displays in devices such as mobile phones, smartphones, and tablet devices, for example. On the other hand, attempts have been made to form the semiconductor layer of the TFT of an oxide semiconductor layer such as IGZO (In—Ga—Zn—O). If forming TFTs of such an oxide semiconductor, it is possible to greatly reduce the off leakage current of the TFTs.
Also, in recent years, in the non-display region of the active matrix substrate, development has been progressing for increasing integration by directly forming driver circuits into a glass substrate included in the active matrix substrate. For example, Patent Document 1 discloses a configuration in which a display device has pixel-driving TFTs formed in the display region of the glass substrate and driver circuit TFTs formed in the non-display region of the glass substrate, and in which the active layer of the pixel-driving TFTs in this display device are made of an oxide semiconductor while the active layer of the driver circuit TFTs are made of low temperature polysilicon.
In the pixel-driving TFTs in Patent Document 1, the oxide semiconductor layer is formed on the surface of an insulating film, and an etching stopper layer is formed on the surface of the oxide semiconductor layer. A source electrode and a drain electrode are formed so as to cover portions of the etching stopper layer and portions of the oxide semiconductor layer. Therefore, when forming pixel-driving TFTs, a metal layer covering the oxide semiconductor layer and the etching stopper layer is etched, and by exposing the etching stopper layer, the source electrode and the drain electrode are formed of the remaining metal layer.